1. Field of the Invention
The present invention relates to a light information device which uses a semiconductor, and a method for producing such an apparatus. More particularly, the present invention relates to a light information device utilizing a photoconductive effect and a photovoltaic effect.
2. Description of the Related Art
In recent years, amorphous materials of IV-group semiconductors such as silicon (Si), germanium (Ge), and carbon (C) are intensively studied, because such materials exhibit a superior photovoltaic effect and a superior photoconductive effect. It has been found that, when such an amorphous semiconductor is doped with an appropriate dose of hydrogen or fluorine, localized states having an energy level in a forbidden band are extremely reduced, and it is possible to obtain a large .mu..tau. product (a product of the mobility of carrier .mu. and a lifetime .tau.; a larger product results in a greater photoconductive effect). Furthermore, the valence electron can be controlled. Accordingly, the amorphous semiconductor with an appropriate dose of hydrogen or fluorine is used for various photoelectric conversion elements.
For example, in amorphous silicon hydride produced by plasma chemical vapor deposition (CVD) (hereinafter referred to as "a-Si:H), the density of localized states is 10.sup.16 eV.sup.-1 cm.sup.-1 or less, and the .mu..tau. product is about 10.sup.-7 cm.sup.2 V.sup.-1. The a-Si:H has an optical bandgap Eg within the range of 1.6 to 1.8 eV, so as to have a photosensitivity in a visible light region. Accordingly, a-Si:H is widely used for the photoelectric conversion element.
Electron-hole pairs caused by photoabsorption are separated at the junction of semiconductor and moved in opposite directions, so as to generate an electromotive force. This is the photovoltaic effect. A typical element which utilizes this effect is a solar cell. Unlike crystalline silicon, the above-mentioned a-Si:H has the following advantages:
(1) It can be formed in a thin film having a thickness of 0.5 to 1 .mu.m (because of its large absorption coefficient). PA1 (2) An extremely small amount of electric power is required for producing a solar cell. PA1 (3) The a-Si:H can be formed on any type of substrate, so that continuous production can be performed.
For these reasons, the a-Si:H is often used for commercially available solar cells. In detail, see "Solar Cell Handbook", (Institute of Electrical Engineers of Japan), Chapter 4.
The electric resistance of a semiconductor is reduced when the semiconductor is irradiated with light while an electric field is applied to the semiconductor. This is the photoconductive effect. Due to this effect, the electrons and holes caused by the photoabsorption can be taken out to the outside through an electrode. Elements utilizing the photoconductive effect include: a light sensor used in an electrophotography, an image sensor, or the like; a spatial light modulator (SLM); an optically addressable display; and the like. For such applications, a-Si:H is increasingly being used, because, in addition to the above-described advantages, a-Si:H has another advantage in that the velocity of response to light is higher compared with other photoconductive materials such as cadmium sulfide (CdS).
A light sensor is described in "Development and Practice of Light-Sensitive Element", NIHON KAGAKU JOHO, Chapter 2, an SLM is described in "Optical Computing", MORIKITA Publisher, Chapter 2, or "Development of Photoconductive Liquid Crystal Light Valve Element using a-Si:H", Pioneer Technical Report, No. 6, pp. 2-9 (1992), and an optically addressable display is described in Japanese Laid-Open Patent Publication Nos. 1-173016 and 4-356024.
The semiconductor amorphous material such as a-Si:H has a reversible photoinducing phenomenon which is called the SW effect (Staebler-Wronski effect). In this phenomenon, long exposure of a-Si:H to bandgap light decreases both the photoconductivity and the dark conductivity thereof, and the photoconductivity and the dark conductivity are restored by annealing at high temperatures. This phenomenon was discovered by Staebler and Wronski in 1977. The SW effect is now described in detail.
FIG. 6 shows the variations of dark conductivity and the photoconductivity in the reversal photoinducing phenomenon which were first measured by Staebler and Wronski. These measured values indicate the case where white light of 200 mW/cm.sup.2 (wavelength of 600 to 900 nm) was used. As is understood from FIG. 6, exposure to light changes the dark conductivity from state A to state B, and the dark conductivity is lower by several orders of magnitude. By the exposure, the photoconductivity decreases by about one order of magnitude. The state in which the dark conductivity is lower by the exposure to light (state B) is stable at room temperature. However, if the temperature is increased to 160.degree. C. or more, the relaxation process is remarkably observed, and the dark conductivity and the photoconductivity are completely restored to the original values within several hours (state A).
In the currently considered dominant opinion, the SW effect is caused because, when the photoinducing electron-hole pairs are non-radiatively recombined via the conduction band tail state and the valence band tail state, defects are generated in the film. For greater detail, see "Reversible conductivity change in discharge-produced amorphous Si", Appl. Phys. Lett. 31, 292 (1977), or "Amorphous Silicon", Ohmsha, Chapter 5, and the like.
It was also reported that the same effect could be attained by continuously applying an electric field to the amorphous material and then injecting carriers into the said amorphous material. For greater detail, see "Influence of excess carriers on the Staebler and Wronski effect of a-Si solar cells", J. Non-cryst. Solids 59&60, 1139 (1983). The reason why the same effect can be obtained is now dominantly considered to be that when the carriers injected in the i-layer of the solar cell are recombined, defects are induced, that cause the film quality of the i-layer to deteriorate. However, the physical mechanism has not yet been sufficiently clarified, and the mechanism is now being intensively studied.
It is known that the amorphous material has (a) a characteristic in that the metastable state is changed by external stimuli such as continuous light irradiation or the application of an electric field, and hence the dark conductivity and photoconductivity are varied with time, and (b) a characteristic in that the dark conductivity and photoconductivity are restored to the original values by annealing at high temperatures.
However, such changes of dark conductivity and photoconductivity with time are not preferable for various light information devices using semiconductor amorphous materials. This is because the characteristics of the apparatus deviate from the initial values as time elapses, so that a satisfactory performance cannot be obtained. Thus, there arises a problem which affects the reliability of the apparatus.
The influence of the SW effect will now be described in detail using an optically addressable display apparatus utilizing the photoconductive effect of a-Si:H as an example.
In an active matrix type display apparatus, active elements (such as transistors and diodes) which are provided for respective pixels are individually driven, so that a display with superior quality can be realized. A TFT-LCD using a thin film transistor as the active element and using liquid crystal as a display medium is typical of such an active matrix type display apparatus. In such a TFT-LCD, when a driving signal is transmitted through electric wiring, the signal waveform is delayed due to the wiring resistance and the parasitic capacitance. Thus, there exists a problem in that a large-screen type display apparatus and a high-definition display apparatus cannot be realized. In addition, in a display apparatus in which electric wiring used for scanning signals and for data signals are both arranged in an X-Y matrix on one and the same substrate, there exits a problem in terms of the process in that electric short-circuit and line breakage may easily occur at the crossings of the electric wirings. In order to solve these problems, an optically addressable display apparatus in which driving signals are optically transmitted has been developed.
FIG. 7 shows an equivalent circuit of a pixel portion of an optically addressable display apparatus described in, for example, Japanese Laid-Open Patent Publication No. 4-356024. In this figure, R.sub.ON and R.sub.OFF indicate an on resistance and an off resistance of a light switching element 30 formed of a-Si:H, and C.sub.LC indicates the capacity of a display medium (e.g., liquid crystal).
In the selected time period T.sub.1, the condition for safely writing 99% or more of the contents of the data signal via the light switching element 30 is represented as follows: EQU 4.6.times..tau..sub.ON =4.6.times.R.sub.ON .times.C.sub.LC &lt;T.sub.1( 1)
where .tau..sub.ON =R.sub.ON .times.C.sub.LC.
In a non-selected time period T.sub.2, if the data signal is leaked from the data line through the light switching element 30, crosstalk occurs. The condition for suppressing the leakage of the data signal within 5% is represented as follows: EQU .tau..sub.OFF /19.5=R.sub.OFF .times.C.sub.LC /19.5&gt;T.sub.2( 2)
where .tau..sub.OFF =R.sub.OFF .times.C.sub.LC.
In the case of a video display using the NTSC system, in general, T.sub.1 =63.5 .mu.sec., T.sub.2 =16.7 msec. (1/60 sec.). It is assumed that C.sub.LC is set to be 1 pF, as a representative example, the following is obtained from Expressions (1) and (2) above. EQU R.sub.ON &lt;1.4.times.10.sup.6 .OMEGA., R.sub.OFF &gt;3.3.times.10.sup.11 .OMEGA.
Thus, the ON/OFF ratio is required to be five orders or more. Expressions (1) and (2) are described in detail in "Liquid Crystal Device Handbook", Nikkan Kogyo Shinbunsha, p. 418.
As described above, the light switching element used in the optically addressable display apparatus is required to set the ON/OFF ratio, i.e, the ratio of the photoconductivity .sigma..sub.p to the dark conductivity .sigma..sub.d to be five orders or more.
The relationship between the light irradiation intensity I and the photoconductivity .sigma..sub.p of a-Si: H is, for example, represented as follows. EQU .sigma..sub.p .varies.I.sup..gamma. exp (-W/k.sub.B T) (3)
where W denotes an activation energy, k.sub.B denotes a Boltzmann coefficient, and T denotes an absolute temperature. It is known that the photoconductivity .sigma..sub.p is in proportion to the .gamma.th power of the light irradiation intensity I. Herein, .gamma. can be a value in the range of 0.5 to 1.
From such a relationship, in order to set the ratio of the photoconductivity .sigma..sub.p to the dark conductivity .sigma..sub.p of a-Si:H to be five orders or more, it is necessary to radiate light with very high intensity. Experimentally, when bandgap light of several tens of mW/cm.sup.2 or more is radiated, the conductivity ratio of five orders or more can be obtained.
Accordingly, the light switching element used in the above-described display apparatus is directly irradiated with light (signal) of several tens of mW/cm.sup.2 or more. As a result, the photoinducing phenomenon of a-Si:H appears as a time elapses, and the characteristics of the light switching element are changed from the initial characteristics. Then, finally the photoconductivity .sigma..sub.p and the dark conductivity .sigma..sub.p do not satisfy the conditions of Expressions (1) and (2). Thus, the deterioration of display performance such as the reduction of contrast and screen flicker becomes very obvious. This is the critical problem in terms of the reliability of the display apparatus.
In the above description, the light switching element used in the optically addressable display apparatus is described. Similarly, in various light information apparatus such as a light sensor utilizing the photoconductive effect of a semiconductor amorphous material, and an SLM, the aging of device characteristics due to the SW effect greatly affects the reliability of the apparatus.
In addition, in the case where a conventional light emitting device is used, it is difficult to increase the intensity of light to be radiated. Therefore, it is difficult to make a margin for the design values of photoconductivity .sigma..sub.p and the dark conductivity .sigma..sub.p so as to satisfy the conditions of Expressions (1) and (2) even as the result of such aging.
In the case of an amorphous solar cell, the stability is currently being improved by modifying the device structure as well as the stability of the film of the semiconductor amorphous material. However, the degradation of conversion efficiency of about 10% cannot be avoided.
In addition, in order to realize a higher definition display apparatus such as high-definition television (HDTV), it is necessary to further improve the ON/OFF ratio of the light switching element by at least one order.